Strict standards exist for particulate and total mercury emissions by coal-fired power plants, petroleum and chemical refineries, coal-fired furnaces, trash burning facilities, incinerators, metallurgical operations, thermal treatment units and other particulate and mercury emitting facilities. These same restrictions apply to mercury vapor, which can enter the atmosphere as a result of low temperature thermal desorption (LTTD) treatment of contaminated soils.
These emission standards exist in order to protect the environment from harmful pollutants. When mercury-containing gases are emitted to the atmosphere, they disperse over a wide area and mercury can be deposited thereby. The dispersed mercury can accumulate in the soil and water supplies, where it can be incorporated into the food chain.
Mercury is extremely harmful to aquatic life and ultimately to the humans who consume mercury-contaminated plants and animals. It is necessary, therefore, to have a safe and effective method of eliminating mercury from the environment.
The problem with capture and treatment of mercury vapor, typically in the context of coal-fired power plants and waste incinerators, has been considered. U.S. Pat. No. 3,193,987 discloses a process of passing mercury-containing vapor over activated carbon impregnated with a metal that forms an amalgam with mercury. U.S. Pat. No. 4,094,777 discloses a process of passing a mercury-containing vapor over an adsorption mass consisting essentially of a support, sulfide copper and sulfide silver. U.S. Pat. No. 3,876,393 discloses a process of passing mercury-containing vapors over activated carbon that has been impregnated with sulfuric acid.
U.S. Pat. No. 3,786,619 discloses a process of passing a mercury-containing gas over a mass containing as an active component, selenium, selenium sulfide or other selenium compounds. Electrostatic precipitators and various filters have traditionally been used for mercury removal, although other apparati have also been disclosed (see e.g., U.S. Pat. Nos. 5,409,522 and 5,607,496).
The problem of recapturing mercury from power plant gaseous streams is analogous to the need for recapturing mercury from incinerators that treat contaminated soils. A process currently in use at soil treatment facilities is known as low temperature thermal desorption (LTTD). LTTD is the main process by which contaminated soils are treated to remove mercury, other metals, and organic contaminants. In this process, contaminated soils are fed into a heating furnace, most commonly a rotary kiln/drum, where the soil is heated by conduction. The heating volatizes the soil components and when a thermal oxidizer is added, the components are oxidized to manageable gases, such as CO2, Cl2, NOX and SOX, where x is 1-3. The hot gas stream is subsequently cooled. The stream may be quenched with water, which cools the stream and concurrently increases the moisture content. Although water quenching is a highly effective cooling method, this treatment increases the difficulty of removing mercury from the gaseous stream. The gaseous stream is further treated to reduce and remove metals, HCl, NOX and SOX using acid scrubbers, carbon beds, condensation units, and the addition of adsorption powders thereto.
When adsorption powders are injected into the gaseous stream, mercury and other metals react to form capturable compounds in the powder; precipitating them from the gaseous stream. The powder-bound mercury is ultimately collected in a bag house for appropriate disposal, while the clean gaseous stream is exhausted to the outside atmosphere. The problem with standard LTTD methods is that some metals, such as mercury, are not efficiently removed from vaporous streams with current technology and will be emitted with the effluent gaseous stream into the environment. Other methods require the use of complex machinery and expensive adsorption beds. LTTD and other methods also suffer from the limitation that mercury removal from high moisture gaseous streams is much more difficult than mercury removal from dry streams.
Available adsorption powders can remove organic compounds, metals, and other contaminants, but they do not effectively remove mercury. For example, one available powder, Sorbalite™, consisting of carbon, calcium hydroxide and sulfur effectively removes HCl from a gaseous stream, but its efficiency of removal is only about 55 to about 65 percent. Another powder, WUELFRAsorb-C™, consisting of alcohol-saturated lime and activated carbon is also inefficient at removing mercury.
Adsorption powders containing sulfur or iodine impregnated on a carbon substrate, at temperatures of about 75° C. or less, showed about 95 percent mercury removal efficiency. However, adsorption powders formulated with sulfur impregnated onto a carbon substrate require gaseous streams to be dry for efficient mercury removal.
Mercury removal efficiency of the adsorption powders described above as well as other available carbon-based powders is known to be temperature dependent. Accordingly, there is a need for an adsorption powder that effectively removes metals and organic compounds from gaseous streams generated by industrial processes and other mercury liberating sources.